1. Field of the Invention
This invention is related to methods for acquiring nuclear magnetic resonance (NMR) measurements for determination of petrophysical properties of formations and properties of fluids therein. Specifically, the invention deals with simultaneous inversion of multiple echo trains for processing and interpreting Nuclear Magnetic Resonance (NMR) log data that exhibit relaxation and/or polarization.
2. Description of the Related Art
Nuclear magnetic resonance is used in the oil industry, among others, and particularly in certain oil well logging tools. NMR instruments may be used for determining, among other things, the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space of earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluids are described, for example, in “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” M. N. Miller et al., Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. Further description is provided in U.S. Pat. No. 5,585,720, of Carl M. Edwards, issued Dec. 17, 1996 and having the same assignee as the present application, entitled “Signal Processing Method For Multiexponentially Decaying Signals And Applications To Nuclear Magnetic Resonance Well Logging Tools.” The disclosure of that patent is incorporated herein by reference.
Deriving accurate relaxation spectra from nuclear magnetic resonance (NMR) data from logging subterranean formations is critical to determining total and effective porosities, irreducible water saturations, and permeabilities of the formations. U.S. Pat. No. 6,069,477 to Chen et al having the same assignee as the present application discusses the constituents of a fluid saturated rock and various porosities of interest. Referring to FIG. 1, the solid portion of the rock is made up of two components, the rock matrix and dry clay. The total porosity as measured by a density logging tool is the difference between the total volume and the solid portion. The total porosity includes clay-bound water (CBW), capillary bound water (also known as Bulk volume Irreducible or BVI), movable water and hydrocarbons. The effective porosity, a quantity of interest to production engineers, is the sum of the last three components and does not include the clay bound water. Accurate spectra are also essential to estimate the irreducible and the movable fluid volumes; distortion of partial porosity distributions that has been commonly observed for a variety of reasons, affects the estimates of these quantities. The reasons for the distortions to occur are mainly due to poor signal-to-noise ratio (SNR) and poor resolution in the time domain of the NMR data.
The most common NMR log acquisition and core measurement method employs T2 measurements using CPMG (Carr, Purcell, Meiboom and Gill) sequence, as taught by Meiboom and Gill in “Modified Spin-Echo Method for Measuring Nuclear Relaxation Time,” Rev. Sci. Instrum. 1958, 29, pp. 688-691. In this method, the echo data in any given echo train are collected at a fixed time interval, the interecho time (TE). Depending on the relaxation rate of the nuclear species under investigation in the underlying system, usually, a few hundred to a few thousand echoes are acquired to sample relaxation decay. Determining a light oil component, which has long relaxation time, requires taking several hundreds of ms of data while determination of CBW, which decays very fast, can be done with echo sequences of as short as a few tens of milliseconds.
There are numerous examples of NMR logging techniques used for obtaining information about earth formations and fluids. In measurement-while-drilling (MWD) operation, measurements are made while the wellbore is being drilled while in wireline logging, measurements are made after a wellbore has been drilled. The logging tools are lowered into the borehole and NMR signals are obtained using different configurations of magnets, transmitter coils and receiver coils. A static magnetic field is produced in the formation using permanent or electro-magnets. The static field aligns nuclear spins within the formation parallel to the static field. A pulsed RF field is applied using a transmitter on the logging tool and the nuclear magnetization signals produced by the pulsed RF field are analyzed to determine formation properties. The prior art shows different radio frequency (RF) pulsing schemes for generating RF fields in the formation. The most commonly used pulsing schemes are variations of the CPMG sequence denoted byTWi,90±π/2,(τ,180,τ,echo)j)i  (1)where TW is a wait time, 90 is a tipping pulse that tips the nuclear spins by an angle substantially equal to 90°, 180 is a refocusing pulse that tips the nuclear spins by an angle substantially equal to 180°, and echo is a spin echo. The time interval between successive refocusing pulses is 2τ, the number of echoes is j, and i denotes repetitions of the basic pulse sequence. A variation of the CPMG sequence is taught in U.S. Pat. No. 6,163,153 to Reiderman in which the use of a refocusing pulse with a tipping angle less than 180° is disclosed.
Rig time is expensive, so that the general objective in wireline logging is to obtain interpretable data within as short a time as possible. In MWD logging, on the other hand, no additional rig time is involved. However, when more measurements can be acquired in a given time, the data quality can be improved. The parameters that may be varied are the acquisition frequencies and the number of different frequencies, the tip angles, the wait time, the number of pulses within a CPMG sequence, and the time interval between the pulses. Long wait times are needed for proper evaluation of formation fluids that have long relaxation times, e.g., gas reservoirs while short wait times and/or short pulse spacings are used for evaluating faster relaxing components, e.g., irreducible fluid (BVI) and clay bound water (CBW). For example, U.S. Pat. No. 6,331,775 to Thern et al., having the same assignee as the present application discusses the use of a dual wait time acquisition for determination of gas saturation in a formation. U.S. Pat. No. 5,023,551 to Kleinberg et al discusses the use of CPMG sequences in well logging. U.S. Pat. No. 6,069,477 to Chen et al teaches the use of pulse sequences with different pulse spacings to determine CBW.
NMR fluid typing applications often involve acquiring multiple echo trains to exploit the diffusion and/or polarization contrasts between the water and hydrocarbon phases. Although proven successful in wells where these contrasts are large, the NMR-based techniques are challenging when the contrasts are small. Carbonates generally exhibit smaller NMR surface relaxivity than clastics, which reduces the relaxation time contrast between movable water and light oil. This difficulty is exacerbated by the small difference in the diffusivities of water and very light oil. In such cases, the methods for processing data are critical and the introduction of a priori information important.
U.S. patent application Ser. No. 10/288,115 of Chen et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, teaches the use of multifrequency NMR acquisition using various combinations of wait times, interecho times in pulse sequences of different lengths in an objective oriented method for formation and reservoir analysis. The objectives of Chen '115 may include formation evaluation, fluid typing, diffusivity determination The pulse sequences taught therein are suitable for use with the present invention. The mention of the Chen '115 application is not intended to be a limitation on the present invention and other types of pulse sequences could be used.
New generation NMR well logging tools can acquire multiple echo trains with different TWs and TEs in a single logging pass. Simultaneously inverting all echo data to obtain the different fluid T2 spectra is a logical method for the processing and interpretation of multiple echo trains acquired with different acquisition parameters. In fact, echo trains acquired with different TEs and TWs can not simply be averaged together to increase the signal-to-noise and inverted because they may exhibit very different T2 decay behaviors. Thus, it is desirable to use an analysis technique that accounts for the different acquisition parameters in the individual echo trains. U.S. patent application Ser. No. 10/435,419 of Chen, having the same assignee as the present invention and the contents of which are incorporated herein by reference, discloses an apparatus and a method of combining echo trains acquired with different parameters. Multiecho sequences are acquired from a first and second region of interest using a first and second radio frequency (RF) pulse sequence. A correction factor depending at least in part on a diffusivity of a fluid in the earth formation is determined, and the first and second multiecho sequences are combined using the correction factor to obtain a combined multiecho sequence. The method taught therein is difficult to extend to multiple echo trains, and the specific problem of small contrasts is not addressed. A similar approach (combining echo trains without gradient variations) is discussed in U.S. Pat. No. 6,377,042 issued to Menger. EP 0886792 to Bonnie et al. discloses a method in which NMR echo signals are acquired with different combinations of TW, TE and field gradient (produced by a gradient coil), and a curve fitting procedure is used to match the resulting signals to a predetermined model. The model itself is limited in scope.
Others have processed multiple echo trains of NMR data by separately inverting them and then by splicing the two separate inversion results. See, for example, U.S. Pat. No. 6,005,389 to Prammer. The methods of splicing generally assume that the ratio T1/T2 is constant.
There is a need for an apparatus and method of inversion of multiple echo trains of NMR data. Such a method should be robust and efficient. The present invention satisfies this need.